﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;

namespace JSLCloud.Utilities.WebApi
{
    public static class ExpressionExtensions
    {
        /// <summary>
        /// lambda表达式扩展
        /// </summary>
        public class ParameterRebinder : ExpressionVisitor
        {
            private readonly Dictionary<ParameterExpression, ParameterExpression> map;

            /// <summary>
            ///
            /// </summary>
            /// <param name="map"></param>
            public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
            {
                this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
            }

            /// <summary>
            ///
            /// </summary>
            /// <param name="map"></param>
            /// <param name="exp"></param>
            /// <returns></returns>
            public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
            {
                return new ParameterRebinder(map).Visit(exp);
            }

            /// <summary>
            ///
            /// </summary>
            /// <param name="p"></param>
            /// <returns></returns>
            protected override Expression VisitParameter(ParameterExpression p)
            {
                ParameterExpression replacement;
                if (map.TryGetValue(p, out replacement))
                {
                    p = replacement;
                }
                return base.VisitParameter(p);
            }
        }

        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // build parameter map (from parameters of second to parameters of first)
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            first = first.Compose(second, Expression.AndAlso);

            return first;
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            first = first.Compose(second, Expression.OrElse);

            return first;
        }
    }
}
